1. Field of the Invention
The present invention relates to an optical glass having a high refractive index and a low dispersion where the refractive index (nd) is not less than 1.75 and an Abbe's number (νd) is not less than 35 and also relates to an optical element such as lens and prism prepared by utilizing this optical glass. More particularly, it relates to an optical glass having a high refractive index and a low dispersion which is suitable as prism and projection lens of optical instruments represented by projector and camera where a highly precise image formation characteristic and also relates to an optical element and an optical instrument prepared therefrom.
2. Description of the Related Art
There is a very high demand for glass having a high refractive index and a low dispersion as a material for optical elements such as various kinds of lenses and, with regard to an optical glass where a refractive index (nd) is not less than 1.75 and an Abbe's number (νd) is not less than 35, various kinds of glass compositions represented by Patent Documents 1 to 3 have been known.
In recent years, there has been a progress in digitization of optical instruments and making them precise and there has been a demand of high properties for optical elements used for the instruments for the reproduction (projection) such as a projector and a projection TV as well as for instruments for taking pictures such as digital camera and video camera. The properties are now not only covering the characteristic such as refractive index, Abbe's number and degree of coloration which have been demanded for optical glass already but also covering little variation in the characteristic under an actually using environment and little environmental load during the manufacture of optical glass and the processing of optical elements.
With regard to a change in image forming characteristic under an actually using environment, it has been presumed to be as follows that an optical element such as lens and prism is fixed by a jig in optical instruments and, when temperature of the using environment changes (such as a change in temperature in the box or use under high temperature), thermal expansion of the optical element is resulted and, due to the difference in its expansion coefficient from that of the fixing jig, stress is resulted in the optical element whereby double refraction is resulted in the optical element and image forming characteristic changes.
As mentioned above, when the image formation characteristic designed by optical constants such as a refractive index and an Abbe's number obtained under predetermined temperature (mostly around room temperature) is not achieved in the actual using environment, there is a disadvantage that, upon the optical designing, the using environment is to be predicted and the design is to be conducted presuming the complicated variations in the characteristic.
When there are components having a high environmental load such as a lead (Pb) compound or an arsenic (As) compound at the time of manufacturing the optical glass and processing of optical elements, there is a disadvantage that special measures are necessary for the prevention of diffusion of polluting substances into air and water. Further, when a rare mineral resource represented by tantalum (Ta) is used in large amounts, not only the production cost becomes high but also cost and labor for recovery of the source are necessary.
With regard to an optical glass with a high refractive index and a low dispersion containing no components having high environmental load in the glass composition, various glass compositions represented by the patent gazettes 1 to 3 are disclosed but no consideration has been done for changes in image formation characteristic under an actually using environment.    Patent Document 1: JP-A-2005-306732    Patent Document 2: JP-A-2002-284542    Patent Document 3: JP-A-2004-161506    Patent Document 4: JP-A-56-160340    Patent Document 5: JP-A-52-14607
Under such circumstances, the an object of the invention is to provide an optical glass having a high refractive index and a low dispersion where the refractive index (nd) is not less than 1.75 and the Abbe's number (νd) is not less than 35 which is hardly affected by image formation characteristic by changes in temperature of the using environment without the use of large amounts of the components having high environmental load and the rare mineral resources.